There are two broad classes of global positioning system (GPS) receivers: code-based and carrier phase based. Code-based receivers are less accurate than carrier phase receivers, but are useful for applications such as mapping. Code-based receivers calculate position based upon the time interval that it takes for signals to travel from at least four satellites to the receiver. More particularly, these receivers develop the pseudoranges to each visible satellite based on the time codes being sent without reference to the carrier phase. A particular rover position is not dependent upon prior positions of the rover; therefore, no memory of what has gone on before is required to calculate individual rover positions.
Carrier phase based receivers are extremely accurate global positioning system (GPS) receivers capable of centimeter-level accuracy. Carrier phase based receivers are typically employed in the fields of surveying and photogrammetry. Carrier phase based receivers calculate distances to visible satellites by determining the number of whole wavelengths and measuring the partial (phase) signal wavelength between the satellites and the receiver's antenna. It is important for carrier phase based receivers to continuously track the number of wavelengths that come across the antenna. Given the number of wavelengths, the pseudo-range is calculated by multiplying the number of wavelengths by the wavelength of the carrier signal. Therefore, these receivers are adversely affected by cycle slips (i.e., discontinuities in the measured carrier beat phase resulting from temporary loss-of-lock in the carrier tracking loop). Further, when performing real-time kinematic (RTK) surveying, a radio link between the base reference station and the rover is critical to the computation of centimeter-level positions at the rover. In some cases, radio outages may cause the loss of RTK positioning. Therefore, it is desirable to detect events such as loss of radio and automatically revert to post process recording when real-time solutions are unavailable.
Turning now to post processed survey systems, generally, only a small subset of data collected during a typical survey is important. For example, in a typical photogrammetry application only the data collected within a predetermined window of time around the activation of the camera is important. Prior post processed survey systems typically perform data logging at the highest rate desired for the entire survey to assure the data of interest (e.g., the window of time around the activation of a camera) will be sampled at the appropriate rate. This prior method of data logging results in very large data files and unnecessarily long post processing times. It is desirable, therefore, to provide a method and apparatus for automatic event recognition to trigger recording changes in a post processed survey system to achieve different logging rates, for example.